1. Field of the Invention
This invention is directed to a communications system which comprises a digital, services-integrating switching equipment and includes two-wire, ISDN transmission (U) interfaces and four-wire, standardized ISDN bus (S) interfaces, and utilizes ISDN terminal equipment that are respectively connected either to an ISDN transmission interface through an ISDN transmission terminating equipment which has layer 1-functions of the OSI reference model for an integrated transmission procedure or to an ISDN bus interface through an ISDN bus terminating equipment which has layer 1-functions of the OSI reference model for an integrated ISDN bus interface, whereby, through a control input provided for this purpose, the ISDN bus terminating equipment is drivable either into an operating mode to obtain the layer 1-functions at the side of the switching equipment or into an operating mode to obtain the layer 1-functions at the side of the ISDN terminal equipment. At the side of the switching equipment, a current source is provided for the remote power supply of the ISDN terminal equipment which is conducted by way of a central office line is coupled directly to the two lines of the two-wire interface or to the phantom circuit for the four-wire ISDN bus interface.
2. Description of the Prior Art
The reference configurations for subscriber lines to services-integrating, digital telecommunications networks--referred to below as ISDN network--are specified in CCITT Recommendation I.411. In accord therewith, an ISDN terminal equipment is brought to a public ISDN switching equipment (ISDN sub-connection) through a central office line and either through a network terminating equipment NTI (main ISDN connection) or via a series circuit of respective network terminating equipment NT1 and NT2. The ISDN base connection of the ISDN terminal equipment occurs by way of a defined ISDN bus interface which is referred to below as a S.sub.o interface, occurring in the network terminating equipment NT1 or, respectively, NT2. The fundamental structure of this four-wire S.sub.o interface is specified in CCITT Recommendation I.412 and the functions to be realized correspond to CCITT Recommendation and I.430. According to CCITT Recommendation I.411, the second network terminating equipment NT2 can be an ISDN private branch exchange or a local network. This means that ISDN terminal equipment are to be generally connected with a S.sub.o interface--to an ISDN private branch exchange as well--in general according to CCITT Recommendation I.430.
Since the distance, about 150 meters, that can be maximally bridged by the S.sub.o interface lines is not adequate to connect more remote ISDN terminal equipment to an ISDN private branch exchange and since the central office lines to the terminal equipment that are predominantly executed in two-wire form in private branch exchange networks prevent a four-wire connection of a S.sub.o interface, these must be brought to the ISDN private branch exchange with suitable transmission equipment through the central office lines that are partly present in two-wire fashion. A special transmission procedure is utilized on the branch central office line for this purpose, this special transmission procedure, utilized in the proximity of the ISDN terminal equipment, conditions a transmission terminal equipment. At the side of the ISDN private branch exchange, the information to be communicated are edited and are transmitted to the relocated transmission terminal equipment through the central office line with the special transmission procedure. The received information are regenerated in this relocated transmission terminal equipment and are shaped such that they meet the conditions of CCITT Recommendation I.430--as already set forth--i.e. that the ISDN terminal equipment can be operated through a S.sub.o interface at the ISDN private branch exchange. In addition to the further transmission terminal equipment, this requires a S.sub.o matching function. This must be added to every remote ISDN terminal equipment or, respectively, to every ISDN terminal equipment which is only connectably by way of a two-wire central office line for connection to an ISDN private branch exchange to the transmission terminal equipment, and is added as an external, complicated line adapter unit--referred to below as PT (private network termination) equipment.
One possibility of avoiding a PT equipment in the respective branch central office line is to integrate the PT equipment functions into the respective ISDN terminal equipment. Such an "U.sub.o -ISDN terminal equipment", for example, can be an ISDN telephone equipment. The "ISDN terminal equipment central office line" interface to be implemented in the "U.sub.o -ISDN terminal equipment" is a transmission system interface. When this transmission system interface is designed as two-wire, then, by analogy to the main ISDN connection, an U-interface that is referred to as U.sub.o -interface at the branch connection results. A number of transmission procedures are known for a two-wire transmission of information through the branch central office lines or, respectively, via the U.sub.o -interface. These are preferably two-wire duplex transmission with echo compensation frequently utilized in public ISDN networks--for example, defined in the FTZ-Richtlinie 1 R 220 for the territory of the Deutsche Bundespost--as well as the time separation transmission mode or, respectively, burst mode preferred in private branch exchange system networks.
An U.sub.o -interface is conducted to a U.sub.o terminating equipment in an U.sub.o -ISDN terminal equipment. This U.sub.o terminating equipment realizes the layer 1-functions according to the OSI reference model as well as further transmission-oriented and electrical functions. In detail, for example, these are the impedance matching of the transmission equipment, the bit and frame synchronization, the power supply feed as well as functions for fault isolation such as, for example, test loops. Within the ISDN terminal equipment, this U.sub.o terminating equipment comprises an internal ISDN interface known in a variety of equipment that is conducted to the further-processing units of an U.sub.o -ISDN terminal equipment. This interface essentially represents a functional boundary between layer 1 and layer 2 of the ISDN signaling protocol for subscriber lines (D-channel protocol).
By analogy to the U.sub.o -ISDN terminal equipment, the S.sub.o interface in an ISDN terminal equipment is conducted to a S.sub.o terminating equipment. The S.sub.o terminating equipment realizes the layer 1-functions for a 4-wire S.sub.o interface. These functions essentially correspond to those of the U.sub.o interface. The S.sub.o interface, however, comprises additional functions for a bus operation of a plurality of S.sub.o -ISDN terminal equipment at a S.sub.o connection--public ISDN network or ISDN private branch exchange. At the exchange equipment side, bus administrators are allocated to the S.sub.o interface and, by contrast, bus user functions are allocated thereto at the side of the ISDN terminal equipment. The differences are in the procedures for access control. Typically, the S.sub.o terminating equipment has access to a control input through which it can be set to an operating mode of the exchange equipment side or, respectively, to an operating mode of the ISDN terminal equipment side. Inside an ISDN terminal equipment, a S.sub.o terminating equipment comprises an internal ISDN interface corresponding to or, respectively, identical to the U.sub.o terminating equipment, this internal ISDN interface being likewise connected to further-processing units of an ISDN terminal equipment.
In order to guarantee, for example, basic voice communication in the ISDN terminal equipment independently of an external energy supply, the ISDN terminal equipment must be supplied with current or, respectively, voltage from the central office line--as in traditional telephone networks as well. To this end--for example, in an ISDN private branch exchange--the current source at the side of the exchange equipment network is directly connected to the two lines with a two-wire U.sub.o interface and, with a four-wire S.sub.o interface, is coupled to a phantom circuit formed for this purpose. A phantom circuit and the coupling of the current source to the phantom circuit are described in detail in CCITT Recommendation I.430.
It is generally valid that an ISDN terminal equipment must be connected to an ISDN switching equipment either through a U.sub.o interface or through a S.sub.o interface depending on the existing ISDN network interface.